1. Field of the Invention
The invention relates to methods pertaining to cell reselection in a cellular telecommunication system. Especially, the invention is related to such a method as specified in the preamble of the first independent method claim.
2. Description of Related Art
For clarification of common terms used in this document, an overview of certain cellular telecommunication system configurations is presented in the following.
Proposals for third-generation systems include UMTS (Universal Mobile Telecommunications System) and FPLMTS/IMT-2000 (Future Public Land Mobile Telecommunications System/International Mobile Telecommunications at 2000 MHz). In these plans cells are categorised according to their size and characteristics into pico-, nano-, micro- and macrocells, and an example of the service level is the bit rate. The bit rate is the highest in picocells and the lowest in macrocells. The cells may overlap partially or completely and there may be different terminals so that not all terminals necessarily are able to utilise all the service levels offered by the cells.
FIG. 1 shows a version of a future cellular radio system which is not entirely new compared with the known GSM system but which includes both known elements and completely new elements. In current cellular radio systems the bottleneck that prevents more advanced services from being offered to the terminals comprises the radio access network RAN which includes the base stations and base station controllers. The core network of a cellular radio system comprises mobile services switching centres (MSC), other network elements (in GSM, e.g. SGSN and GGSN, i.e. Serving GPRS Support Node and Gateway GPRS Support node, where GPRS stands for General Packet Radio Service) and the related transmission systems. According e.g. to the GSM+ specifications developed from GSM the core network can also provide new services.
In FIG. 1, the core network of a cellular radio system 930 comprises a core network CN 931 which has three parallel radio access networks linked to it. Of those, networks 932 and 933 are UMTS radio access networks and network 934 is a GSM radio access network. The upper UMTS radio access network 932 is e.g. a commercial radio access network, owned by a telecommunications operator offering mobile services, which equally serves all subscribers of said telecommunications operator. The lower UMTS radio access network 933 is e.g. private and owned e.g. by a company in whose premises said radio access network operates. Typically the cells of the private radio access network 933 are nano- and/or picocells in which only terminals of the employees of said company can operate. All three radio access networks may have cells of different sizes offering different types of services. Additionally, cells of all three radio access networks 932, 933 and 934 may overlap either entirely or in part. The bit rate used at a given moment of time depends, among other things, on the radio path conditions, characteristics of the services used, regional overall capacity of the cellular system and the capacity needs of other users. The new types of radio access networks mentioned above are called generic radio access networks (GRAN). Such a network can co-operate with different types of fixed core networks CN and especially with the GPRS network of the GSM system. The generic radio access network (GRAN) can be defined as a set of base stations (BS) and radio network controllers (RNC) that are capable of communicating with each other using signaling messages. Below, the generic radio access network will be called in short a radio network GRAN.
The terminal 935 shown in FIG. 1 is preferably a so-called dual-mode terminal that can serve either as a second-generation GSM terminal or as a third-generation UMTS terminal according to what kind of services are available at each particular location and what the user's communication needs are. It may also be a multimode terminal that can function as terminal of several different communications systems according to need and the services available. Radio access networks and services available to the user are specified in a subscriber identity module 936 (SIM) connected to the terminal.
A conventional way to treat a connection setup request in a congestion situation is to simply reject the connection setup request. Thereafter the mobile station (MS) needs to find a new cell for a new connection attempt. The problem with this approach is a high amount of failed signalling, if the mobile station needs to make several connection attempts to several different cells before finding a cell, which accepts the new connection. Such a situation may arise, when many of the surrounding cells are congested as well. The setup of the connection can therefore also take a long time, which may frustrate the user of the mobile station.
One other prior art way of treating a connection setup request congestion situation is to perform the connection setup as requested by the mobile station, and directly thereafter force a handover to a less congested cell. Such a method is quite satisfactory from the viewpoint of the user of the mobile station, since the network accepts the new connection quickly, but such a method causes too much signalling in the form of handover signalling.
At the time of writing this patent application, the RRC specifications of third generation cellular networks the network has no explicit control over which cell the UE selects after an RRC connection setup is rejected in the serving cell. It has also been a problem in the GSM system that after the rejection of signaling connection request, cell reselection and reattempt actions of mobile stations have not been under control of the network.